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Depending on how many molecules come together, a reaction can be unimolecular, bimolecular or even trimolecular. The kinetic order of any elementary reaction or reaction step is equal to its molecularity, and the rate equation of an elementary reaction can therefore be determined by inspection, from the molecularity. [1]
It breaks down an apparently unimolecular reaction into two elementary steps, with a rate constant for each elementary step. The rate law and rate equation for the entire reaction can be derived from the rate equations and rate constants for the two steps. The Lindemann mechanism is used to model gas phase decomposition or isomerization reactions
The Rice–Ramsperger–Kassel–Marcus (RRKM) theory is a theory of chemical reactivity. [1] [2] [3] It was developed by Rice and Ramsperger in 1927 [4] and Kassel in 1928 [5] (RRK theory [6]) and generalized (into the RRKM theory) in 1952 by Marcus [7] who took the transition state theory developed by Eyring in 1935 into account.
The unimolecular nucleophilic substitution (S N 1) reaction is a substitution reaction in organic chemistry. The Hughes-Ingold symbol of the mechanism expresses two properties—"S N " stands for " nucleophilic substitution ", and the "1" says that the rate-determining step is unimolecular .
The E1cB mechanism is just one of three types of elimination reaction. The other two elimination reactions are E1 and E2 reactions. Although the mechanisms are similar, they vary in the timing of the deprotonation of the α-carbon and the loss of the leaving group. E1 stands for unimolecular elimination, and E2 stands for bimolecular elimination.
An apparently elementary reaction may be in fact a stepwise reaction, i.e. a complicated sequence of chemical reactions, with reaction intermediates of variable lifetimes. In a unimolecular elementary reaction, a molecule A dissociates or isomerises to form the products(s)
Such reactions require the addition of energy in the form of heat or light. A typical example of a unimolecular reaction is the cis–trans isomerization, in which the cis-form of a compound converts to the trans-form or vice versa. [15] In a typical dissociation reaction, a bond in a molecule splits (ruptures) resulting in two molecular fragments.
Entropy of activation determines the preexponential factor A of the Arrhenius equation for temperature dependence of reaction rates. The relationship depends on the molecularity of the reaction: for reactions in solution and unimolecular gas reactions A = (ek B T/h) exp(ΔS ‡ /R), while for bimolecular gas reactions A = (e 2 k B T/h) (RT/p ...